K'Nex Fruit Machine (Mark 3)




Introduction: K'Nex Fruit Machine (Mark 3)


The aim was to produce a machine which was smaller and lighter than its two predecessors, K'Nex Fruit Machine (Mark 1) and K'Nex Fruit Machine (Mark 2).

Like the previous versions, the machine was designed as the construction progressed. It is not practicable to provide a step-by-step sequence of its construction owing to its complexity, and the purpose of this 'Instructable' is to present the various K'Nex techniques which have been used as well as giving a description of the machine. It will hopefully inspire some K'Nex owners to be creative rather than just following instructions.

It was intended that the machine would be portable, but if one person were to attempt to lift it, a subsequent appointment with a chiropractor would probably be required. It was also intended to be a desktop version, but the desk would have to be quite strong. The dimensions are 26" (66cm) wide, 36" (91cm) deep and 42" (107cm) high (to the top of the fascia - 53" (135cm) to the top of the slot). It is much more compact than the previous versions.


Box Structure

A box structure has been used for the frame (see the 12th picture). This method produces very strong adjoining cubes by using opposite diagonal rods which are at right-angles to each other. This structure cannot be used near the reels because they need room to spin, and so the right-hand side of the machine is formed from a large array of blue-rod-sided cubes, giving rigidity to the whole machine. The base is formed from red-rod-sided cubes, giving a sound footing for what is a heavy construction.

K'Nex Ratchets

There are two K'Nex ratchets in the machine (part numbers 91307 & 91290): one for the game timer and one for the damping of the pay-out cradle.

The placing of the ratchet in the gear train needs some consideration. If the ratchet is installed too near the driving gear, there will be too much movement in that gear before the ratchet engages, but if it placed too far down the gear train, too much force will be required for it to work properly. Only by trial is the best position found, because it depends on the driving force and the nature of the flywheel.

In the game timer, the gear train has a red gear (34 teeth) driving a blue gear (14 teeth). On this second shaft is another red gear which meshes with a blue gear on a third shaft. It is on this third shaft that the ratchet has been placed. There are two more red gears in the train, ending with a flywheel (i.e. a white connector with eight blue rods around its edge, with a 1" wheel at the end of each one - see the 8th picture).

In the cradle-damping gearbox, which also uses red and blue gears, the ratchet worked best on the 4th shaft.

It might seem a good idea to oil the ratchet to make it run more smoothly. Don't. What happens is that the oil makes the two components stick together and it doesn't work at all well (it's very jerky).

Attaching a Rod to a Connector

If a rod is attached to a straight two-way connector, it is quite easy to pull it out (in the direction of the rod). If the same rod is attached to a connector which has a slot on either side of the rod, it needs quite a lot of force to remove it because the other slots make the join tighter. If, however, a green rod (say) is inserted in each slot on either side of the rod, it is extremely difficult to pull the rod out. This technique has been used wherever a strong join was required. See the 15th (last) picture.

Falling Balls

K'Nex balls are made from halves which push together. If they bang into each other or hit anything hard, the halves can come apart, either slightly (the usual case) or completely.

When an inserted ball falls against a full ball reserve, it bounces down a 'stagger-tube' which breaks its fall as it lands in the 'cash'-box. This 'stagger-tube' is illustrated in the 14th (penultimate) picture, but there are many other ways of achieving the same effect.

Colour Scheme

Most of the K'Nex pieces come in standard colours, but many box kits use a variety of other colours. These can be quite difficult to come across in any quantity. There being enough of the alternatively-coloured pieces, a combination of mainly black and grey components has been used on the front of the machine so that it resembles an old mechanical one-armed bandit, albeit somewhat larger. At first glance it is not obvious that it has been made from K'Nex.

Combining Rods

If a certain length of combined rods is required, a lot of trial and error can be involved to find the best combination. At the end of these notes there is a link to a PDF file which gives a useful table.

Looking After Your Pieces

Most of the parts used in this construction are second-hand, having been bought from eBay or at car boot (trunk) sales. At the end of these notes there is a link to another PDF file which explains how to look after your pieces.


The stages were as follows, with the framework being built as the stages progressed:

  • The reels
  • The reel spinners
  • The reel brakes
  • The handle
  • The handle guide
  • The handle damper
  • The mechanism which spins the reels
  • The game-timer cam
  • The game-timer gearbox
  • The game-timer reset
  • The pay-out cradle
  • The pay-out cradle lock and release mechanism
  • The pay-out mechanism
  • The pay-out cradle damper
  • The slot
  • The handle lock
  • The pay-out reserve chutes
  • The pay-out delivery chute
  • The pay-out tray
  • The 'stagger-tube'
  • The cash-box
  • The reel symbols
  • The fascia
  • The awards and instructions cards


The handle-lock is released when a ball is inserted.

When the handle is pulled, the following actions take place:

  • The handle lock is re-engaged
  • The game-timer is reset
  • The handle-damping gearbox is reset
  • Any balls won in the previous game are replenished from the reserve
  • The pay-out cradle is reset
  • The reels are spun

When the handle is returned:

  • The game timer starts
  • The first reel stops
  • The second reel stops
  • The third reel stops
  • The pay-out cradle is released
  • If there is a winning combination showing on the reels, the corresponding number of balls are released

The game lasts around 11 seconds from the insertion of the ball to the delivery of any win.


No rubber bands or electric motors have been used. The machine uses a weight to help return the handle after it has been pulled, another weight to power the game timer, and two light weights to encourage the reel-brakes to settle on the reels. There are also various counterweights. The spin of the reels receives power from the downward pull of the handle.


Well, it's not actually a slot, it's a hole...

If the handle is pulled without a ball having been inserted, it will lock (unless it is pulled really hard, in which case it will fall to pieces). The lock is released when a ball is inserted. This can be seen in the video from 2m 1s - the lock - near the top right - can be seen to move just after the ball has been inserted, and return just after the handle has been pulled.

Inserted balls roll into the pay-out reserve, and when this is full (it holds 39 balls) they fall down a 'stagger-tube' into a cash-box (OK - the coin denomination is balls, but you get the drift...).


The timer is required to control the game sequence after the handle has been pulled. It turns a long cam which releases the three reels one at a time and then releases the pay-out cradle. The timer uses a gearbox which is powered by a weight (formed from four tyred medium wheels) and uses a K'Nex ratchet.

The cam stretches across all three reels and has been constructed out of white connectors which are held together by white rods. Every white connector (apart from the end ones) has at least two white rods clamping it to the previous and next one, the white rods being staggered along the length. In order to make sure that the white rods are gripped as tightly as possible, green rods have been placed in every empty white-connector slot. The resulting unit is long, strong and resistant to torsion. An example of the method of construction (without any green rods) can be seen in the 13th (antepenultimate) picture.

White rods work better than any of the other lengths because the protruding ends fit between the white connectors whilst gripping the connectors tightly along the length of the cam.

The cam is quite difficult to construct because many of the white rods have to be inserted into two adjacent white connectors' slots at the same time. The secret is to use another rod to push the white rod in place by pushing down along its length.

The machine's cam can be seen in action in the video from 3m 52s.


Much of the size and weight in the previous two machines were due to the chunkiness of the components, particularly the handle which had to lift a heavy weight.

In this machine, the handle is light and flimsy, and as a result has to have a guide so that it doesn't bend too much. The disadvantage is that is cannot support the heavy weight that would be required to return it to its starting position. The player is therefore expected to help the handle to return - this is not an issue because it is instinctive anyway. A gearbox with a flywheel is used to damp the handle's return. The weight which helps to return the handle consists of ten tyred large wheels, piled up on a grey rod.


In the previous two incarnations of the machine, the reels were constructed without regard to their final weight, requiring quite a large force to spin them, and heavy brakes to slow them down.

To make the reels as light as possible in this version of the machine, the number of connectors on the outside of the reels is minimal, the symbol cards being used to provide some rigidity. The symbols have not been laminated, again to reduce the weight.

The reel brake consists of connectors instead of a wheel, thus reducing the required width, again helping to reduce the weight.

The width of each reel assembly is that of a red rod (plus its connectors). In the first fruit machine the width was that of joined red, blue and white rods, and in the second that of joined red and green rods.

These smaller, closer-together reels have enabled a narrower machine to be built.

In the previous versions of the machine, a heavy weight was raised when the handle was pulled, and then it was released, the force being used to spin the reels. This made the components chunky and heavy, increasing the size and weight of the machine.

In this machine it is the downward force of the handle which is used to spin the reels. It has the disadvantage that a gentle pull will produce hardly any spin, but in practice players tend not to do this - but hey, this is a fun construction, not a commercial gaming machine!

Sometimes (not very often) a reel stops in between symbols. In previous versions of the machine an unexpected pay-out might have resulted, but with this one the offending reel will be nudged into position if it would otherwise result in an ambiguous size of win.


After the reels have stopped, a pay-out cradle is lowered to test the depths of the aligned slots in the reels, the deeper the shallowest slot the higher the pay-out. The cradle is lowered gently (by using a gearbox with a K'Nex ratchet) so that the shock does not upset the pay-out release. It is important that the cradle does not bend or distort (otherwise the pay-out could be erratic) and this is why it is quite heavy and robust, albeit finely balanced.

Balls are released by allowing them to roll down a chute. There is a barrier at the end of the chute, and other barriers between the second and third, fourth and fifth, and eighth and ninth balls (see the 10th picture).

If the win is two balls, the cradle (which is pivoted along its middle) is lowered onto the reels so that the end barrier is lifted high enough to release two balls, but not so high that the barrier between the second and third balls is lifted too.

If the win is four balls, the cradle is lowered a little more so that the first two barriers are lifted.

If the win is eight balls, the barrier between the fourth and fifth balls gets raised as well.

The barrier between the eighth and ninth balls only gets raised at the start of each game, when any paid-out balls are replenished. When this is lifted, the second and third barriers are lifted too, the first barrier remaining where it is so that no balls roll out.


The main source of the randomness comes from the force with which the handle is pulled - the greater the force, the faster the reels spin.

The second main source comes from the delay between pulling the handle (which starts the reels spinning) and returning it, because the game timer starts only when the handle has been returned.

In practice, the combination of the symbols at the end of a game is unpredictable.


The symbols on each reel, and the awards, are:

Reel 1    Reel 2    Reel 3
   8         8         8                          A W A R D S
   2         4         0
   0         2         2         888 = 8 balls    488 = 4 balls    2** = 2 balls
   8         0         0         884 = 4 balls    484 = 4 balls    *2* = 2 balls
   4         8         0         848 = 4 balls    448 = 4 balls    **2 = 2 balls
   8         4         4         844 = 4 balls    444 = 4 balls
   2         0         0	
   4         2         2         where * is a symbol with red lines, i.e. not 0

and so the number of symbols on each reel is:

 Symbol   Reel 1    Reel 2    Reel 3
    8        3         2         1
    4        2         2         1
    2        2         2         2
    0        1         2         4
             -         -         -
  Total      8         8         8

The number of ways of winning 8 is therefore 3x2x1 = 6

The number of ways of winning 4 is the number of ways of getting 4 or 8
on each of the three reels minus the number of ways of winning 8,
i.e. 5x4x2 - 3x2x1 = 34

The number of ways of winning 2 is the number of ways of getting 2, 4 or 8
on each of the three reels minus the number of ways of winning 4 or 8,
i.e. 7x6x4 - 5x4x2 - 3x2x1 = 128

There are 8x8x8 = 512 combinations, and so the chance of a win is
(6 + 34 + 128) / 512 = 0.33

and the expected return is
(6x8 + 34x4 + 128x2) / 512 = 440 / 512 = 85.9%

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    6 years ago

    I love it could you make one even smaller? :0 I made a ball counter I think is right up your street with dials and levers I think you'd like it



    Reply 4 years ago

    You may like to know that I've just finished a smaller one: No Instructable yet.


    Reply 7 years ago

    Thank you!

    Lucas The Boss
    Lucas The Boss

    7 years ago

    Wow, this is solidly mechanical, no motors. That my friend is very very impressive. May I ask if you were a mechanical engineer before you started building with knex? Favorited and I subscribed!


    Reply 7 years ago on Introduction

    Thanks for that.

    I've had a collection of over 30 slot machines for many years, and this machine (and its two predecessors) use the same pay-out principle as was used in the Beromat machine of the 1950s (pictured).


    Dang, this is very impressive! Outstanding job, the attention to detail is superb and automation with motors for this type of machine would be complicated, let alone no motors at all!


    Reply 7 years ago on Introduction

    How nice of you to say! Thank you.

    I'm toying with the idea of a yet smaller version, but I'm not sure that would be possible. Mind you, I thought that after the first version...


    7 years ago

    Oh what I would do to have my own,


    7 years ago on Introduction

    That is one awesome machine! My favorite part about it is that it uses no motors. I also like the color scheme you chose.


    Reply 7 years ago on Introduction

    Thanks, Shadowman.

    When the machine was 'finished', I had used mainly white connectors for the front and the pay-out tray, but it looked rather tacky.

    Since I had enough black connectors and rods, and grey pieces too, I decided to buy some black panels and change the look so that it resembled a mechanical machine.